The present invention relates generally to the field of heat transfer and, in particular, the present invention relates to thermal management of electronic devices.
In one embodiment, the present invention is used to transfer heat generated by electronic devices or groups of devices, such as transistors, as are commonly included on integrated circuit (IC) chips. A brief discussion of some electronic systems using IC""s, such as personal computers or lower level computer components, is included below to show some possible areas of application for the present invention.
IC""s are typically assembled into packages by physically and electrically coupling them to a substrate made of organic or ceramic material. One or more IC packages can be physically and electrically coupled to a printed circuit board (PCB) to form an xe2x80x9celectronic assemblyxe2x80x9d. The xe2x80x9celectronic assemblyxe2x80x9d can be part of an xe2x80x9celectronic systemxe2x80x9d. An xe2x80x9celectronic systemxe2x80x9d is broadly defined herein as any product comprising an xe2x80x9celectronic assemblyxe2x80x9d. Examples of electronic systems include computers (e.g., desktop, laptop, hand-held, server, etc.), wireless communications devices (e.g., cellular phones, cordless phones, pagers, etc.), computer-related peripherals (e.g., printers, scanners, monitors, etc.), entertainment devices (e.g., televisions, radios, stereos, tape and compact disc players, video cassette recorders, MP3 (Motion Picture Experts Group, Audio Layer 3) players, etc.), and the like.
In the field of electronic systems there is an incessant competitive pressure among manufacturers to drive the performance of their equipment up while driving down production costs. This is particularly true regarding forming electronic devices such as transistors in IC""s, where each new generation of IC must provide increased performance, particularly in terms of an increased number of devices and higher clock frequencies, while generally being smaller or more compact in size. As the density and clock frequency of IC""s increase, they accordingly generate a greater amount of heat. However, the performance and reliability of IC""s are known to diminish as the temperature to which they are subjected increases, so it becomes increasingly important to adequately dissipate heat from IC environments.
An IC is fabricated on a semiconductor substrate that may comprise a number of metal layers selectively patterned to provide metal interconnect lines (referred to herein as xe2x80x9ctracesxe2x80x9d), and one or more electronic devices attached in or on one or more surfaces of the semiconductor substrate. The electronic device or devices are functionally connected to other elements of an electronic system through a hierarchy of electrically conductive paths that include the substrate traces. The substrate traces typically carry signals that are transmitted between the electronic devices, such as transistors, of the IC. Electronic devices and traces can be configured in an IC to form processors. Some IC""s have a relatively large number of input/output (I/O) terminals (also called xe2x80x9clandsxe2x80x9d), as well as a large number of power and ground terminals or lands.
As the internal circuitry of IC""s, such as processors, operates at higher and higher clock frequencies, and as IC""s operate at higher and higher power levels, the amount of heat generated by such IC""s can increase their operating temperature to unacceptable levels. Thermal management of IC""s refers to the ability to keep temperature-sensitive elements in an IC within a prescribed operating temperature. Thermal management has evolved to address the increased temperatures created within such electronic devices as a result of increased processing speed/power of the electronic devices.
FIG. 1 illustrates a cross-sectional representation of a common configuration IC package 30. IC package 30 represents a typical structure that includes an IC die 40 mounted in xe2x80x9cflip-chipxe2x80x9d orientation with its active side facing downward to couple with lands 52 on the upper surface of a board 50 through solder balls or bumps 42. Board 50 can be a one-layer board or a multi-layer board, and it can include additional lands 54 on its opposite surface for mating with additional packaging structure (not shown).
Die 40 generates its heat from internal structure, including wiring traces, that is located near its active side; however, a significant portion of the heat is dissipated through its back side. Heat that is concentrated within the die is dissipated to a large surface that is in contact with the die in the form of an external heat spreader 60 that is typically formed of metal such as copper or aluminum. To improve the thermal conductivity between the die and the external heat spreader 60, a thermal interface material 70 is often provided between the die and external heat spreader 60. To further dissipate heat from external heat spreader 60, a heat sink 80 optionally having heat fins 82 is often coupled to external heat spreader 60. Heat sink 80 dissipates heat into the ambient environment. The thermal interface material 70 shown in FIG. 1 is intended to be a generic illustration of any thermal interface material. In the following discussion, the thermal interface material 70 and its thickness 72 are used to describe many embodiments of a thermal interface material, including several embodiments of the invention.
Prior thermal interface materials 70 have included an oil matrix containing conducting particles such as metal or aluminum nitride (AlN). The combination of matrix and particles forms a viscous material or grease. Conducting particle shapes can most accurately be described as granular, or spherical in shape.
A smaller IC die 40 with a thinner layer of thermal interface material 70 is required to meet industry size reduction pressures. Thermal interface material 70 is applied to the die and spread to a very thin thickness 72 using hydraulic pressure. This pressure must be high, in order to spread the viscous thermal interface material 70. However, it cannot be so high as to damage the more delicate, reduced size IC die. Higher viscosity thermal interface materials 70 in turn require higher pressure to spread them to a given thickness 72. Therefore, a maximum thermal interface material viscosity for use with a given die at a given thickness 72 can be defined. The viscosity must be low enough to allow the thermal interface material 70 to be spread to the design thickness 72 without using an application pressure that would damage the die.
Currently, to obtain increased thermal conductivity, the loading percentage (calculated as a weight percentage of the particles to the total weight) of conductive particles is merely increased. However, this approach may only be used up to a certain loading percentage. Higher loading percentages increase the viscosity of the existing thermal interface materials 70. At a high enough loading percentage, the matrix becomes excessively thickened by the conductive particles, and the thermal interface material 70 will no longer spread to the desired thickness under acceptable pressures.
Another approach to enhance existing thermal interface materials is to use expensive high thermal conductivity materials such as silver or diamond powder. While this approach increases the thermal conductivity of the thermal interface material 70, it significantly increases the cost.
Thermal conduction of existing thermal interface materials 70 is therefore limited by the thermal properties of existing particle materials, and the acceptable range of loading percentages available under design constraints. What is needed is a low cost thermal conducting material with higher thermal conductivity for a given low viscosity.